fix: public api surface fixes + stability markers

- ts-api-guardian will now error if a new public symbol is added with a stability marker (`@stable`, `@experimental`, `@deprecated`)
- DomEventsPlugin and KeyEventsPlugin were removed from public api surface - these classes is an implementation detail
- deprecated BROWSER_PROVIDERS was removed completely
- `@angular/compiler` was removed from the ts-api-guardian check since this package shouldn't contain anything that users need to directly import
- the rest of the api surface was conservatively marked as stable or experimental

BREAKING CHANGES: DomEventsPlugin and KeyEventsPlugin previously exported from core are no longer public - these classes are implementation detail.

Previously deprecated BROWSER_PROVIDERS was completely removed from platform-browser.

Closes #9236
Closes #9235
Ref #9234

modules/@angular/forms/src/directives.ts

@@ -73,7 +73,10 @@ export const FORM_DIRECTIVES: Type[] = /*@ts2dart_const*/[
   RequiredValidator, MinLengthValidator, MaxLengthValidator, PatternValidator
 ];
 
+/**
+ * @experimental
+ */
 export const REACTIVE_FORM_DIRECTIVES: Type[] =
     /*@ts2dart_const*/[
       FormControlDirective, FormGroupDirective, FormControlName, FormGroupName, FormArrayName
-    ];
+    ];